In recent years, an optical phase modulation method such as a DP-QPSK (Dual Polarization Differential Quadrature Phase Shift Keying) method, for example, has been attracting attention in an optical transmission system. The DP-QPSK method is a method producing less deterioration during transmission and capable of achieving an increased capacity. The standardization thereof has been underway by the OIF (Optical Internetworking Forum).
An example of an optical modulator employing such an optical phase modulation method is an optical modulator which performs modulation with a semiconductor chip. Accurate optical coupling between an optical fiber and the semiconductor chip is expected for the optical modulater employing the semiconductor chip while it is capable of reducing its size when compared with an optical modulator employing a ferroelectric crystal such as lithium niobate (LiNbO3), for example.
More specifically, whereas the beam diameter of a single-mode optical fiber is about 10 μm, for example, a beam diameter to be incident on the semiconductor chip is about 1 to 2 μm, for example. Thus, if a relative position between the optical fiber and the semiconductor chip is displaced, the accuracy of the optical coupling is reduced, thereby increasing the coupling loss thereof. Specifically, if a positional displacement amount between the optical fiber and the semiconductor chip is 0, the coupling loss therebetween is also 0 as indicated in FIG. 9, for example. If the positional displacement amount is about 0.3 μm, for example, on the other hand, the coupling loss of −1 dB is generated.
Since high-accuracy optical coupling is expected for the optical modulator employing the semiconductor chip as described above, it is desirable that the position of the optical fiber be fixed accurately. Specifically, when the optical fiber is fixed with solder, for example, it has been considered that the position of the optical fiber is measured with a laser micrometer so as to fix the optical fiber at an accurate position.
Patent Document 1: Japanese Laid-open Patent Publication No. 2004-157208
However, since solder has a melting point of at least 100° C. or higher, the temperature of the solder to be melted and solidified when fixing the optical fiber greatly differs from the temperature of an operating environment for the optical modulator which operates at a room temperature, for example. Thus, during the operation of the optical modulator, the position of the optical fiber fixed with the solder varies along with a volume change due to thermal expansion of the solder. Specifically, the solder contracts as it cools down after the solidification thereof, possibly resulting in a displacement in the position of the optical fiber fixed with the solder in the operating environment of the optical modulator.
In view of this, when the optical fiber is fixed with the solder, a method such that the positional variation due to the thermal expansion of the solder is measured in the course of fixing the optical fiber, then the solder is melted again, and the position of the optical fiber is adjusted is employed. Thus, there is a problem in that the number of steps is increased in order to fix the optical fiber with the solder and achieve high-accuracy optical coupling between the optical fiber and the semiconductor chip.